Upgrading plugin to version 0.8.01.

Most notable change: upgrading from GamesClient/PlusClient/AppStateClient to GoogleApiClient. For other changes, see CHANGELOG.txt.
Adding UPGRADING.txt.
Update README.md
Update UPGRADING.txt
Fix bug in OnSignInFailed

Fixed wrong parameter for the call of "showErrorDialog" in com.google.example.games.pluginsupport.SignInHelperManager
Merge pull request #24 from antonlicht/master

Fix bug in OnSignInFailed
Update README.md
Merge branch 'master' of https://github.com/playgameservices/play-games-plugin-for-unity
* Making it compatible with the latest version of googleplay service lib (rev 15).
* Changing the logging strategy: The main activity holding the only one GameHelper object. This avoid loosing data, like the match passed by the notification. Make it also simpler to debug.
* Adding ways to retrieve selected game by the InboxUI.
* Should handle multi accounts one device, and properly handling logging changes due to game notifications.

Still work in progress